Statistical data of Mendelian inheritance in man demonstrates, up to now, that 1660 single gene disorders have been identified and 989 disease-related genes have been identified before Jun. 30, 2000. Most recessive genetic disorders may be treated by introducing the normal gene into cells of the patient. With the advancement of research, gene therapy regarding dominant genetic disorders and somatic cell-related tumors has begun. In 1995, E. Marshall put forward in Science that the key point of gene therapy research was novel vector development and discovery, but the problem of an effective gene therapy vector remain unsolved so far. The main reason is that the researchers do not step out of the circle of constructing a vector using viral components. General speaking, commonly-used viral vectors have many defects as follows: 1) Instability: gene insertion efficiency is low, and the vector exists in the cell nucleus partially as a form of an attached body that is not inherited stably during cell division. As a result, the therapeutic gene cannot be expressed for a long time and in a stable manner. 2) Poor safety: for example, mutation caused by random insertion may affect function of normal genes at integration sites, and even activate an oncogene, which may result in other diseases and tumors respectively. Furthermore, viral vectors may generate wild-type recombinant virus with replication ability and so harm the patient. In recent years, it was reported that an adenovirus vector caused a patient death during the conduct of a gene therapy. 3) Immunogenicity: the proteins produced by viral genes and protein contaminating the vector during purification could induce an immunogenic reaction and influence the expression of the therapeutic gene.
In the middle of the 1980's, a gene targeting vector was developed based on the principle of homologous recombination to achieve site-directed integration, which could avoid immunogenicity and random integration, but the gene targeting vector used for site-directed repair in gene therapy and replacement of a defective gene have to utilize specific fragments of the two sides of the gene as targeting sequences, therefore its application is limited and the transfection efficiency is still low. In fact, it is useful for gene knock-out in embryonic stem cells and fertilized egg cells, but is not suitable for site-directed integration in mature somatic cells (Galli-Taliadoros L A, Sedgwick J W, Wood S A, et al. J Immunol Meth 1995, 181:1-15; Hasty P, Rivera-Perez J, Chang C, et al. Mol Cell Biol 1991, 11(9):4509-4517).
Rosenbery concluded that no case had shown certain clinical efficacy among hundreds of gene therapy experiments (Rosenbery L E & Schechter A. N. Science, 2000; 287:1751). Thus, development of a novel, stably inherited gene vector causing no harm to a human body remains a key problem to be solved.